Current commercial systems for the consolidation of products using pressure and heat involve the use of massive hydraulic presses based on heat transfer by conduction. Such presses are equipped with thick press platens or plates of high mass and thermal capacity, which are heated by steam or heating oils, passing through a labyrinth of interconnected passageways within the platens. High mass and thermal capacity of the platens is necessary for storing sufficient heat to prevent excessive cooling by cold materials deposited into the press for consolidation. In addition, the pressing platens must be thick also to provide sufficient rigidity which is required to prevent bending deformations of the platens caused by uneven distribution of material to be consolidated over the internal working area of the platens.
The loading of such press platens using conduction heat transfer and open pressing can be viewed as a case where the platens are acted on in a direction perpendicular to the plane of the platen from one side by a nonuniformly distributed load, and from the other side by a nonuniformly distributed elastic support in reaction to the pressure from the first side. Because the distribution of loads and supports is random and may be quite variable, high bending moments may be created which cause a significant deformation of platens during pressing and thereby causing variable thickness of the pressed products. Because such variations cannot be tolerated in commercially produced composite products, the press platens are made 2.5 to 7 inches thick depending on the product.
It has been recognized in the prior art that injecting steam into composite materials during consolidation by pressure and heat produces several improvements, the main one of which is an increase in the curing rate of thermosetting resin adhesives used to consolidate the materials. Several systems have been proposed for this purpose. For example, Futo U.S. Pat. No. 3,619,950 has proposed a gas-tight envelope made of Teflon sheets reinforced in a suitable manner and surrounding press platens with pressed products between them, for the purpose of controlling the ambient atmosphere in and around the products.
Corbin U.S. Pat. No. 3,295,167 shows a steaming apparatus for consolidation of composite products, the apparatus comprising a source of superheated steam which is fed into a plate having a steam chamber and a plurality of spaced openings from such chamber and through which the superheated steam is passed into the product being pressed. The steam passes through and out of the open pressed product to speed up the heat transfer and curing of thermosetting resins.
The patent to Shen, U.S. Pat. No. 3,891,738 discloses press platens which have a chamber and aperture openings on the surface adjacent to the products to be pressed. Steam passes from one press platen through the pressed products into another press platen lying opposite the product, thereby speeding up curing of thermosetting resin adhesives.
The Nyberg patent, U.S. Pat. No. 4,162,877 shows, instead of two, one almost identical press platen as that of Shen with a chamber and aperture openings on the surface coming into contact with the pressed product. Steam is injected from the press platen through the openings into the pressed board and released through the same openings back into the platen after the curing of the thermosetting resin in the pressed product.
All of these aforementioned systems, however, use steam primarily to warm the product being pressed, and the press platens are used for heat transfer simultaneously by conduction, i.e. the products become heated not only from the injected steam by convection, but also from the press platens themselves by conduction in accordance with conventional practice. These devices are accordingly an offshoot of the current commercial systems described above which employ relatively massive presses; therefore, such dual function platens of the aforediscussed patents are too complicated and heavy and too expensive to replace and clean when necessary. In addition, in presses such as shown by Corbin, the steam used is not trapped but is permitted to escape, thereby losing heat and losing control of the adhesive or curing by virtue of uncontrolled steam flow.
In the production of thick products of low and medium density from poor thermal conductors such as wood, fiberglass or porous plastic materials, heat transfer is a major problem. Consolidation times using heat transfer by conduction, which is almost used exclusively in commerce in the present time, are too long and represent a significant cost item.
Another problem which exists in the art relative to wood chips is the loss of heat in the chipping and drying operation. After chipping, the wood particles comprise about 50% water which is far too much for conventional procedures for making particle board and the like; therefore, the wood particles, e.g., fibers, are normally heated to about 400.degree.-450.degree. F. to effect drying thereof. It would be desirable to provide a system in which wetter than normal wood particles can be used, thereby reducing the amount of drying necessary and saving energy.